You're right to point out the seeming contradiction. Here's why it's not exactly a contradiction, but a compromise based on existing conditions and risk mitigation



The Contradiction You See

The NEC permits something it also states can be dangerous. That seems like a clear conflict.

Why it's not a strict contradiction (and why it's allowed)



Legacy Systems (Existing Installations)

The NEC recognizes the immense cost and disruption of requiring homeowners to rewire existing homes that have 3-wire range circuits. Imagine millions of homes needing new wiring. That's a huge logistical and financial burden. Instead, the code makes allowances for these legacy systems. It's a grandfathered exception.

Risk Assessment and Mitigation

The NEC is constantly evolving. Early versions of the code didn't fully understand the potential hazards of shared neutral/ground connections. As understanding grew, the code was updated to prevent this in new installations. However, completely outlawing existing 3-wire connections immediately was deemed impractical. It's assumed that the risk in these existing systems is "acceptable" until renovations make a full upgrade more manageable. The risk in a brand new installation isn't acceptable, so the 4-wire system is required.

Safety vs. Practicality

The NEC attempts to balance optimal safety with practical considerations. Completely eliminating every possible electrical hazard instantly is impossible. Retrofitting every existing home is not feasible or affordable.

Analogy

Think of older cars without airbags. We now know airbags are safer. Modern cars must have them. But we didn't suddenly scrap every car on the road when airbags became standard. The older cars are allowed to remain in use, but all new cars must meet the higher safety standard.

In summary

The NEC isn't contradicting itself so much as applying different standards based on:1.

Whether it's a new installation or an existing one.2. Balancing safety risks with practical limitations (cost, disruption).3. Acknowledging the historical context of the original wiring practices.

The goal is always to increase safety over time, but in a way that's realistically achievable. Therefore, the 4-wire requirement in new construction is in place to completely eliminate the possibility of shared neutral/ground hazards, while the allowance of 3-wire systems in existing installations is a temporary compromise. Flag for review